Measurement of Ferromagnetic Slabs Permeability Based on a Novel Planar Triple-Coil Sensor

Eddy current methods are wildly used for measuringthe electromagnetic property of ferromagneticsamples due to its high sensitivities and portability (when using the remote electromagnetic sensor). However, in diverse applications, the eddy current measurement is always hampered by the lift-off variation - the gap between the sensing probe and the sample. In this paper, the measurement error of the permeability caused by the lift-off variation could be significantly reduced when using a developed planar triple-coil sensor. An inverse algorithm, the single-frequency eddy current algorithm is established for the evaluation of the permeability by referring to the phase of themeasured impedance or inductance. Based on the experiments of several ferromagnetic samples when using the developed sensor and inverse algorithm, the measured permeability is proved can be almost immune to the lift-off variations.